1. Field of the Invention
The present invention relates to a method for evaluating metal contamination of a silicon single crystal grown by the Czochralski method (hereinafter referred to as “CZ” method), particularly to the method for evaluating metal contamination of a silicon single crystal used to recognize a metal impurity generation state in a pulling apparatus, which becomes a factor of silicon single crystal metal contamination.
2. Description of the Related Art
The silicon single crystal is used as raw material for the substrate of a semiconductor device. There are various methods for producing the silicon single crystal. Among others, there is widely adopted the CZ method in which the silicon single crystal is grown by pulling the silicon single crystal from a melt as raw material in a quartz crucible.
FIG. 1 schematically shows a configuration of a pulling apparatus suitable to the silicon single crystal growing by the CZ method. As shown in FIG. 1, a crucible 2 is disposed in a central portion of a chamber 1 constituting an outer frame of the pulling apparatus. The crucible 2 has a dual structure, that is, a quartz crucible 2a which is located inside, and a graphite crucible 2b which is located outside of the quartz crucible 2a and in which the quartz crucible 2a is fitted. The graphite crucible 2b is fixed to an upper end portion of a supporting shaft 3, and the crucible 2 is axially moved up and down through a ascending/descending drive of the supporting shaft 3 while circumferentially rotated through a rotatary drive of the supporting shaft 3.
A resistance heating type heater 4 is disposed outside the crucible 2 in such a manner as surrounding the crucible 2, and further a thermal insulation sleeve 5 is disposed outside the heater 4 along an inner surface of the chamber 1. The heater 4 causes a polycrystalline silicon material filled in the crucible 2 to melt down, thereby forming a raw material melt 6 in the crucible 2.
On the other hand, a crystal suspending member 7 such as a wire is disposed above the crucible 2 coaxially with the supporting shaft 3. The crystal suspending member 7 is axially moved up and down while rotated by a pulling mechanism (not shown) provided in an upper end portion of the chamber 1. A seed crystal 8 is attached to a lower end portion of the crystal suspending member 7. The seed crystal 8 is dipped in the raw material melt 6 in the crucible 2, and the seed crystal 8 is gradually moved up while rotated, thereby growing a silicon single crystal 9 below the seed crystal 8.
A neck portion, a shoulder portion, a cylindrical portion, and a tail portion are sequentially formed in the growth of the silicon single crystal 9. The neck portion is located immediately below the seed crystal 8. In the shoulder portion, a diameter of the silicon single crystal 9 is gradually increased to a required diameter. The cylindrical portion having the required diameter is converted to a silicon wafer as end-product. In the tail portion, the diameter is gradually decreased from the diameter of the cylindrical portion in order to prevent the introduction of the dislocation at the final growth stage.
A graphite heat-shielding member 10 surrounding the silicon single crystal 9 in the middle of pulling-up is provided above the crucible 2 in order to efficiently grow the silicon single crystal 9 free from grown-in defects such as COP and a dislocation cluster. A water-cooling member 11 made of metal such as copper, which surrounds the silicon single crystal 9, is provided in a lower portion of the inside of the heat-shielding member 10.
Electric cables are connected to the crystal suspending member 7 and the supporting shaft 3 from a power supply device 12. The power supply device 12 applies a voltage between the crystal suspending member 7 and the supporting shaft 3, so that the voltage can be applied between the silicon single crystal 9 and the crucible 2 with the raw material melt 6 interposed therebetween.
For example, Japanese Patent Application Publication No. 2003-12393 describes that the single crystal is well grown while the voltage ranging from −50V to +50V is applied between the silicon single crystal and the crucible. Therefore, impurities negatively charged in the raw material melt can be removed from a crystal growth interface into the raw material melt by applying the voltage of −50V to 0V onto the crystal suspending member side, while the metal impurities positively charged in the raw material melt can be removed from the crystal growth interface into the raw material melt by applying the voltage of 0V to +50V onto the crystal suspending member side, so that the generation of the dislocation in the single crystal due to impurities is prevented in growing the single crystal.
The metal impurities contained in the silicon single crystal, particularly heavy metals such as iron, copper, and nickel cause deterioration of a withstand voltage of an oxide film or an increase in leak current after the silicon single crystal is sliced and processed into substrates for a semiconductor device, thereby deteriorating performance of the semiconductor device. The contamination of the silicon single crystal by such metal impurities derives from the polycrystalline materials and various hot-zone components constituting the pulling apparatus, such as the crucible, the heater, the heat-shielding member, and the water-cooling member.
The components constituting the pulling apparatus deteriorate as they are used, and metal impurities are diffused with the deterioration of the components to allow the metal impurities to be taken in to the silicon single crystal during the process for growing the single crystal. Therefore, the silicon single crystal is contaminated by the metal impurities.
In order to grow the high-quality silicon single crystal, it is important that components constituting the pulling apparatus and polycrystalline materials be controlled so as not to allow the metal contamination to occur in the silicon single crystal. Conventionally, a sample wafer is cut out in each of the grown silicon single crystals, and the metal contamination of the sample wafer is sequentially evaluated to grasp time-series behavior how metal impurities are generated in the pulling apparatus, which is a factor in metal contamination, whereby the components of the pulling apparatus are controlled. The metal contamination of the sample wafer is evaluated by either lifetime measurement, an analysis by means of a Surface Photo Voltage (SPV) method, or an analysis by means of a complete dissolution method.
As disclosed in Japanese Patent Application Publication No. 9-218175, in the lifetime measurement, a rate of change of a recombination lifetime of carriers in the sample wafer relative to the time how long the sample wafer is aimed by a laser beam is obtained for the sample wafer collected from the p-type silicon single crystal in which boron is used as a dopant by a Microwave Photo Conductivity Decay (μ-PCD) method, and the presence or absence of iron is detected by the rate of change. When a calibration curve indicating the recombination lifetime is produced in each iron concentration by a Deep Level Transient Spectroscopy (DLTS) method, the iron concentration can be obtained based on the calibration curve.
With reference to the measurement performed by the SPV method, as disclosed in Japanese Patent Application Publication No. 2005-64054, a surface of the sample wafer collected from the p-type silicon single crystal is activated by light shining or heating, a diffusion length of a minority carrier is measured immediately after an Fe—B pair is disassociated by the activation and the diffusion length of the minority carrier is also measured when Fe and B are returned to the Fe—B pair again, and the iron concentration is measured from the diffusion lengths.
With reference to the measurement performed by the complete dissolution method, as disclosed in Japanese Patent Application Publication No. 2007-227516, part of the sample wafer collected from the p-type silicon single crystal is dissolved in a mixed acid of a hydrofluoric acid and a nitric acid, and the concentration of the metal such as the copper in the solution is measured using an atomic absorption spectrophotometer or an inductively coupled plasma-mass spectrometer.
However, in the case where the metal impurity concentration contained in the silicon single crystal, i.e. the sample wafer, is equal to or smaller than the limit of detectability, the metal contamination cannot be evaluated by the lifetime measurement, an analysis by means of the SPV method, and an analysis by means of the complete dissolution method. That is, the metal contamination cannot be evaluated until the metal impurity concentration in the silicon single crystal reaches detectability limit or more, and the time-series behavior of the metal impurity generation in the pulling apparatus cannot be grasped. Therefore, the components of the pulling apparatus are insufficiently controlled, and there is a risk such that the silicon single crystal having tremendous metal contamination may abruptly appear.